The present invention relates to cellular mobile telephone systems. More specifically, the present invention relates to system and method for controlling closed loop transmitter power in a code division multiple access (CDMA) cellular mobile telephone system.
CDMA is a modulation and multiple access technique based on spread spectrum communication that has been applied to improve capacity in digital cellular radio communication systems. Code division multiple access using direct sequence (for convenience, herein called CDMA) is one of several techniques for facilitating communication in which a large number of system users are present. In the CDMA system all the users employ the same code for encoding and decoding of their respective information sequences, the transmitted signals in the common spectrum are distinguished from one another by superimposing a different pseudo-random pattern, also called a code, on each transmitted signal. Thus a particular receiver can receive the transmitted information intended for it by knowing the pseudo-random pattern, i.e., the key used by corresponding transmitter.
The cellular mobile channel typically can be characterized by two separate phenomena, average path loss and fading. The average path loss can be described statically by log-normal distribution whose mean is proportional to inverse fourth-power of the path distance. The second phenomenon has Rayleigh fading characteristic. The Rayleigh fading process is caused by the physical environment, and results in copies of the signal arriving simultaneously from many directions with different transmission delays. This causes significant phase differences between the paths with the possibility for destructive summation, resulting in deep fades. Fading is very disruptive to the channel, which results in poor communications. While Rayleigh fading may be independent for forward (cell-to-mobile) links, log-normal shadowing normally will exhibit reciprocity.
Power control is very important system requirement for a CDMA system, since only by controlling of power of each user resources can be shared equally among users and capacity maximized. In order to maximize the capacity of CDMA system in terms of the number of simultaneous calls in given system bandwidth, the transmitted power of each mobile unit is controlled so that its signal arrives at the cell site with the minimum required signal-to-interference ratio. Power control is essential in any CDMA system in order to mitigate the "near/far problem" preventing users that are geographically closer to the base station from "overpowering" users that are farther away. Furthermore, the nature of fading channels cause power variation that must be compensated if it is possible. To equalize the received powers, a combination of open and closed loop is used. The goal of the open loop is to adjust transmitted power according to changes in received power. For reverse link open loop, the mobile stations measure the received power level from the cell sites and adjust their transmitter power in an indirectly proportional manner, attempting to have all mobile station transmitted signals arrive at the cell with the same nominal power level. The open loop control can cope with the very slow shadow type fading.
For the reverse link closed loop power control, the base station measure the relative received power level or more precisely measures EbIo (the ratio of signal energy per bit to the interference power spectral density, Io) of each of associated mobile station's and compare it to an adjustable threshold. A determination is made to transmit a power-up command or a power-down command to the mobile station. The power adjustment command signals the mobile station to nominally increase or to decrease the mobile station transmit power by a high enough to permit tracking of slow Rayleigh fading, approximately 1000 commands per second. The power adjustment command is sent to a mobile station in the forward channel addressed to the mobile station. The mobile station combines the received adjustment commands with an open loop estimate to obtain the final value for transmit radiated power.
The goal of the closed loop is to provide rapid corrections to the open loop estimate in order to maintain the optimum transmit power. This closed loop correction accommodates gain tolerance and unequal propagation losses between the forward and reverse links. The variations in relative path losses and shadowing effects will generally be slow enough to be controlled. The slow Rayleigh fading could be controlled too, however, the variations associated with Rayleigh fading could be too rapid to be tracked by power control. It is known that the effectiveness of the combination of interleaving and coding in combating the effects of power variation due to slow Rayleigh fading is reduced. At low speeds (slow fading) the power control reacts to compensate for fading. The power control and interleaving/coding are most effective in complementary parameter regions, thus providing a degree of robustness for both fast and slow Rayleigh fading. The closed loop power control is the crucial component to combat slow Rayleigh fading. Another benefit of power control is that each user transmits only as much energy as is required, thus prolonging battery life in portable transmitters.
The BER/FER (bit/frame error rate) performances in a CDMA system are directly related to the closed loop power control efficacy in combating the effects of received power variation. Further, by controlling the power, unnecessary system interference is minimized, increasing overall system capacity. The accuracy of EbIo measurement for the purpose of closed loop power control is essential for CDMA cellular system performance, so that receiver could overcome deleterious fading, providing a required degree of robustness. The EbIo measurement for low signal to interference ratios suffers high degradation and introduces errors in power control. For precise power control accurate and reliable EbIo measurements are required.
In a coherent detection data communication system, the known pilot symbols are usually transmitted with data symbols. In the receiver, the transfer function of channel is estimated using the pilot symbols, and data symbols are detected based on the estimated transfer function. The same pilot symbol is used for EbIo measurements. In order to minimize losses caused by transmission of pilot symbols, the ratio of the transmitted pilot symbols to the transmitted data symbols is usually low, and using only pilot symbols for EbIo measurement could not always satisfy requirements for accurate EbIo measurements.
Assuming that the receiver is working in a region of reasonable symbol errors (slightly less than 0.5), and by introducing data symbols decisions in the process of EbIo measurements, the accuracy of power estimation could be increased resulting in an improvement of closed loop power control performance. It is important that the latency in determining and transmission of power adjustment commands (close loop delay) is minimized, so that channel conditions will not change significantly before the responding of mobile unit. For these reasons we could not use the re-encoding of decoder's output data for EbIo measurement, since the usual long delay due to decoding/deinterleaving is inconsistent with the need for fast power control. The power control command is sent "unprotected" using the forward link for the same reasons.
Therefore, the hard decisions before deinterleaving/decoding with high error rate have to be used for EbIo measurements.
By including data symbols decisions in the process of EbIo measurements, the accuracy of power estimation is increased, resulting in an improvement of closed loop power control performance. The tracking of the received power variation is improved. Also, the receivers' BER performance are improved and deviation of received signal power has been reduced. However, for low EbIo values and due to variation of received power, particularly during deep fades, when the rate of wrong decisions become large, the use of data decisions for EbIo measurement introduces difficulties in controlling the transmitted power and for some conditions leads to the total blocking of the signal. Thus, the using of data symbol decision for EbIo measurement should be controlled and selective.